(12-05) 11:56 PST SAN FRANCISCO -- People who have had certain types of bariatric, or weight-loss, surgery may be at increased risk of acetaminophen poisoning and, in turn, liver failure, according to a small study at San Francisco's California Pacific Medical Center.

Researchers found that among 54 patients who had suffered acetaminophen-induced liver failure over a three-year period, 17 percent had had weight-loss surgery. That's a surprisingly high percentage, considering that less than 1 percent of the general population has had the surgery, said Dr. Timothy Davern, a liver disease and gastroenterology expert at CPMC.

"The over-representation of bariatric patients was really shocking," he said. "Obesity is such an important public health problem, and bariatric surgery is highly effective. But if our data are correct, and we think they are, then there should probably be special warnings about the potential for liver toxicity from acetaminophen."

The researchers looked at 101 cases of acute liver failure seen at California Pacific Medical Center, more than half of which were caused by acetaminophen poisoning. Among the nine patients who had had weight-loss surgery, all of them had liver failure caused by acetaminophen overdose. They had all had the most common type of weight-loss surgery, which involves closing off and bypassing a large section of the stomach and part of the intestinal tract.

More study needed

The findings will need to be reproduced in a larger study involving multiple medical centers, the researchers said, and weight-loss surgeons agreed. At this time, there is no reason for bariatric surgery patients to be alarmed, and they should continue using acetaminophen if that's their preferred pain medication or their doctor has prescribed it.

But acetaminophen poisoning is a problem in the United States as a whole, liver experts noted, and it might be prudent for patients who have had weight-loss surgery to make doubly sure they're not exceeding maximum recommended doses of the drug.

Acetaminophen is more commonly known as Tylenol in the United States, but the drug is also found in more than 200 prescription and over-the-counter medications, including narcotics Vicodin and Percocet along with NyQuil and Sudafed.

Because many patients aren't aware of all the products that contain acetaminophen, they can be at risk of accidentally overdosing. Acetaminophen poisoning sends more than 50,000 people to the emergency room every year, according to the U.S. Food and Drug Administration. It's the most common cause of acute liver failure in the United States.

The FDA recommends that people limit their daily acetaminophen use to 4,000 milligrams a day - a dosage that many physicians argue should be lower, and the FDA is considering changing.

"Tylenol misuse is chronic in this country," said Dr. John Morton, director of bariatric surgery at Stanford. "We recommend Tylenol to our gastric bypass patients at this point. But I'm a big believer in making sure patients understand their medications and making them aware of the potential risks-benefits."

Looking for causes

Still, Morton and other weight-loss surgeons said they wouldn't be surprised to learn that their patients had some increased sensitivity to acetaminophen, and for a variety of possible reasons.

Obesity very often causes liver damage, and while some of that damage can be reversed after people lose a lot of weight, it's possible that weight-loss surgery patients still have weakened livers that are more susceptible to harm from acetaminophen.

Also, patients who undergo gastric bypass are at higher risk of developing ulcers and are told to stay away from many over-the-counter pain medications like aspirin and ibuprofen. Therefore, they're left with only prescription narcotics and acetaminophen for pain relief, which may be a reason they're more likely to take too much of the drug.

What's less clear is whether something about the surgical procedure itself makes patients more susceptible to acetaminophen poisoning. Gastric bypass surgery can have multiple effects on how people process food and beverages - most patients have to take a multivitamin, for example, because their bodies aren't able to absorb certain nutrients - and that may apply to acetaminophen too.

Some studies have shown that acetaminophen levels tend to spike higher and faster in the livers of people who have had bariatric surgery than those who haven't, although that doesn't mean they're more likely to suffer negative effects from the drug.

Morton at Stanford said he couldn't think of anything "intrinsic to the surgery itself" that would cause patients to be more vulnerable to acetaminophen overdoses. Davern and Holt wondered if multiple, subtle changes in the digestive tracts of people who have had the surgery cause the liver to metabolize acetaminophen differently.

"We actually may never know in a detailed way what the enhanced sensitivity is," said Davern, adding that the cause is not as important as confirming that there is, in fact, a problem, and then getting the word out to patients and their doctors.

Glad he had surgery

Gary Stein, 66, underwent bariatric surgery at California Pacific Medical Center four years ago and has lost more than 200 pounds since then, and maintained the lower weight. He said the surgery was life-changing - he no longer needs to take medication for high blood pressure or diabetes, and whereas he used to struggle to walk more than 100 feet at a time, he can now go for "miles and miles."

So he doesn't regret having the surgery at all, but Stein also said he wouldn't be surprised to learn that he may be at increased risk of acetaminophen poisoning. Stein said he doesn't take the medication often, although it's the only one he uses regularly since his doctor told him to stop using aspirin and ibuprofen.

"It makes sense to me," said Stein, a retired physician who lives in Walnut Creek. "You're already injured and then you take acetaminophen, which is totally broken down in the liver, and that makes the liver work even harder. You could have really deleterious results in this population."